Ball bearing assemblies are used to carry loads in a variety of applications. For example, such an assembly may be used to raise and lower (or move backward and forward) a platform or other structure, and/or to rotate the platform or other structure. Additionally, in some applications both the inner and outer raceways of the assembly move reciprocally. Numerous configurations of ball bearing assemblies for a wide variety of applications are known.
Referring to FIG. 1, ball bearing assembly 100 is shown. Ball bearing assembly 100 comprises shaft 101, ball cage 102 and housing 103. In assembly 100, shaft 101 moves in an up and down (Z) direction as shown by arrow 110. Shaft 101 is, for example, cylindrical, as is the inner surface of housing 103. Ball cage 102 is disposed in the annular space between shaft 101 and housing 103. Ball cage 102 retains a plurality of balls loosely in place circumferentially around the cage. In assembly 100, cage 102 holds 2 `rings` of balls, 105a and 105b. In each ring there may be any number of balls depending upon the dimensions of the components of the assembly and the required support. Additionally, more than the two rings 105a and 105b as shown in FIG. 1 may be employed. Also, it will be appreciated that the balls 105 do not necessarily need to be arranged in rings as shown. Rather, the position of the balls may be staggered, to provide maximum surface contact with the shaft 101. The Z movement means 107, which could be, for example, a lead screw, is used to move the shaft 101 up and down. At the top of shaft 101, platform 116 is shown. Platform 116 could be used, for example, to hold a semiconductor wafer upon which some operation is to be performed. For example, after the semiconductor die have been fabricated on the wafer, it is often required to probe each die, before the wafer is cut to produce chips to be packaged.
As shown in FIG. 1, the gap between shaft 101 and housing 103 is greater than the diameter of the balls 105. This is known as a clearance fit. The appropriate clearance fit is desirable because it allows the shaft to move within the housing 103 without undue wear. However, a problem arises in that unwanted motion of the shaft 101 can occur during use. Generally this motion is a tilting motion as shown by arrow 115. Because there is this unintended motion in the ball bearing assembly 100, it may be difficult to align precisely an object held on the platform to another object, for example, a semiconductor die to a probe. Additionally, even if alignment is completed, forces occurring during use may prevent the desired operation from being completed. For example, in the probing of a semiconductor wafer, when the outer edge of the wafer is probed a force shown as arrow 117 at the edge of the wafer is applied. When the force 117 is applied, the platform 116 will tilt down where this force is applied. Additionally, the force may also cause X, Y and rotational motion. These motions will reduce the probing accuracy and may prevent the probing of the die. The tilting motion will prevent some of the probes from accurately contacting the die, and the X, Y, and rotational motion will cause the die to be out of alignment with the probe tips as well.
FIG. 2A shows bearing assembly 200, which is one possible solution to this problem. In this case, balls 205 have a diameter that is greater than the annular space between shaft 201 and housing 203. In this case, the balls 205 penetrate, to some degree, both shaft 201 and housing 203. Additionally, the balls 205 deform slightly. This is known as an interference or preload fit. In such a fit, the shaft is tightly held within the housing. Therefore maximum shaft stiffness is provided, which would minimize platform deflection under forces, such as force 117 of FIG. 1. In FIG. 2A, the platform 216 is in an up position. FIG. 2B shows assembly 200 in a down position. As can be seen from the figure, as the shaft 201 moves up and down, ball cage 202 with balls 205 moves up and down as well, due to the tight fit of the balls.
FIG. 2C shows a problem which occurs with such an arrangement over time. After many up and down motions ball cage 202 has a tendency to migrate in either an up or a down direction. In FIG. 2C ball cage 202 has migrated all the way to base 207. As shown, shaft 201 still has a distance 215 which it can travel in the downward direction. As shaft 201 moves down, ball cage 202 cannot move with it. Because this is an interference fit, balls 205 cannot roll as shaft 201 moves down. In some cases, this will prevent shaft 201 from moving downward. In other cases, the shaft 201 can still move, but the balls do not roll with it. Instead, the shaft 201 slides over the contact surface of the balls 205, with high friction. This results in increased wear of the shaft 201, the balls 205 and possibly the housing 203, and therefore a shorter life of or damage to the assembly 200. Because of this wear, manufacturers of ball bearing assemblies commonly recommend a clearance fit. Typically, there will also be some means (e.g., platform 116) for stopping ball cage 202 near the top of assembly 200, so that upward migration of ball cage 202 will cause the same problem as downward migration.
What is needed is a ball bearing assembly which suffers little or no deflection under the forces that occur during use. The assembly should prevent significant migration of the ball cage which leads to the problems described above. What is further needed is an assembly which allows for increased life by reducing wear of the parts. Any such assembly should also maintain these advantages over an extended temperature range of operation.